Ionization-type smoke sensors are known and are recognized as being effective for providing an early warning of the presence of smoke or products of combustion. As illustrated in FIG. 1 a known sensor 10 includes an active chamber electrode 12a which is positioned adjacent to a reference chamber electrode 12b. Disposed between the electrodes 12a and 12b is an intermediate or sensing electrode 12c. The unit 10 includes minute quantities of radioactive material which function as a source of ions in a known fashion.
It is also known to test such detectors by applying a test voltage to the active chamber electrode 12a. This test voltage can be applied via a test switch TS which can be automatically or manually operated, in combination with resisters R1 and R2.
FIGS. 2A and 2B are graphs illustrating application of a test voltage to the chamber via the test switch TS, as well as the response of the intermediate electrode 12c to the applied test voltage. As the graphs in FIGS. 2A and 2B illustrate while the applied test voltage may be a pulse having relatively very short rise and fall times the voltage response CEV of the intermediate electrode 12c has exponential rise and fall times due to large impedance values associated with ionization sensors, as well as stray capacitances.
As a result of the exponential rise and falls times of the voltage CEV of the intermediate electrode 12c there is a delay between when the test voltage is applied by the test switch TS and when its effects can be detected at the output of the intermediate electrode 12c. This delay in turn imposes upper limits on how fast the test function can be carried out.
Speed of execution of the test function becomes particularly important in modern fire alarm systems which may include hundreds of ionization-type sensing units coupled to a central control panel via common communication lines. In such instances the central control panel may on a regular basis test some or all of the sensing units. Where the system includes three or four hundred detectors delays which are insignificant with just a few detectors become highly undesirable.
Thus, there continues to be a need for ionization-type smoke sensors which have shorter response times. Preferably response times in the test mode could be shortened without significantly increasing the cost of the sensors.
Additionally, there continues to be an ongoing problem with the response of known smoke detectors in the presence of varying velocities of ambient air. Most modern commercial or industrial facilities include building-wide heating/cooling systems which regularly circulate the air through the building and condition it so that it remains comfortably warm in the winter and comfortably cool in the summer.
The forced movement of the ambient atmosphere, and the resulting velocity thereof, is known to have a negative impact on the performance of smoke detectors. Due to the movement of air it may take longer for the smoke density to increase to a sufficient level so as to produce a potential alarm condition.
There thus continues to be a need for detectors which do not exhibit an inordinate drop in sensitivity as a function of air velocity. Preferably compensating for air velocity could be achieved also without materially increasing the cost of the detector.